This proposal has a single objective - to determine how the presence of a viable embryo in the uterus signals its presence and causes maintenance of a functional corpus luteum. The cow has been chosen as the experimental animal, primarily because implantation does not occur in this species until long after the embryo has caused maintenance of the corpus luteum of pregnancy. Three experiments are planned. The first is designed to produce evidence for either production of a fetal gonadotrophin that overcomes normal cyclic luteolytic mechanisms, or alternatively for an "anti-luteolytic" mechanism that inactivates the normal cyclic luteolytic processes. If this experiment provides evidence for a fetal luteotrophin, a second experiment will be conducted to measure the production of the fetal luteotrophin during pregnancy, to determine the tissue responsible for its production, and to determine the route by which it reaches the corpus luteum. If, on the other hand, the first experiment provides no evidence for a fetal luteotrophin, studies of the exact mechanism by which the embryo interferes with cyclic luteolytic mechanisms will be carried out. These studies will involve measurements of arachidonic acid and prostaglandins in endometrial tissues and uterine venous blood of pregnant and nonpregnant animals, of lipoprotein lipase and other enzymes capable of increasing free arachidonic acid concentrations in corpora lutea of pregnant and nonpregnant animals and of prostaglandin synthetase activities in luteal tissues of pregnant and nonpregnant animals.